Methods and systems for controlling fiber polarization mode dispersion (PMD)

ABSTRACT

A method is provided for predicting an installed performance parameter of an optical fiber cable. The method includes obtaining a measurement indicative of a value of the performance parameter at a first moment in time. A measurement indicative of a value of the performance parameter at a second moment in time may then be obtained. A first correlation may then be determined between the measurement at the first moment in time and the measurement at the second moment in time. A value of the performance parameter at the second moment in time may then be estimated based upon the measurement at the first moment in time in combination with the first correlation, the first correlation being based upon observations of a manner in which the performance parameter varies over time for at least a second optical fiber.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication No. 60/541,929, filed Feb. 6, 2004, the entirety of which isincorporated by reference herein.

FIELD OF THE INVENTION

Systems and methods consistent with principles of the invention relategenerally to optical fiber cables and, more particularly, to controllingpolarization mode dispersion (PMD) in optical fiber cables.

BACKGROUND OF THE INVENTION

With the explosion in communication via the Internet in recent years,there has been a corresponding increase in demand for high-speedbandwidth, such as that provided by optical fibers. In fiber opticcommunication systems, a fiber that carries optical signals containsasymmetries. These asymmetries result in the optical properties of thefiber not being the same in all directions. Thus, the fiber isbirefringent, where the material displays two different indices ofrefraction. This fiber birefringence causes polarization mode dispersion(PMD).

PMD is measured like a vector quantity, where a differential group delayis the magnitude of the vector and the principal state of polarization(PSP) is the direction. There are two PSPs associated with PMD. The twoPSPs propagate at slightly different velocities with the distribution ofsignal power varying with time.

PMD is a time varying stochastic effect. PMD varies in time with ambienttemperature, fiber movement, and mechanical stress on the fibers.Compensating for PMD can be difficult because of the time varying natureand randomness of PMD.

PMD has been shown to be an impairment to the transmission of signalsover telecommunication optical fiber at line rates of 10 Gbits/s orabove over long distances. Though the problem originates in both opticalcomponents and the transmission fiber, the ongoing focus has been PMDreduction in the fiber.

Current processes enable designing and manufacturing opticaltransmission fiber with very low values of PMD. However, not allmanufacturers have access to the intellectual property which is criticalto successfully and consistently produce fiber with good PMDperformance. Hence, the optical fiber market offers a wide spectrum ofquality with little differentiation in specifications. A currentchallenge for optical cable manufacturers and installers is assessingthe true PMD quality of the fiber based on information provided by thefiber manufacturer. In particular, a common question is what fiber/cablequalification procedure should be followed to assure good PMDperformance in the installed system.

The traditional specifications on PMD have focused on the link designvalue (LDV) or maximum differential group delay (DGD-max). These metricshave an inherent weakness, however, of being virtually impossible for acustomer to verify. It has become apparent that more information on PMDis required, specific to a customers' fiber order and cable type.Acquiring this information involves careful attention to measurementtechniques and correlation of fibers as they move from draw towersthrough the final installed cable product.

Accordingly, there is a need in the art of optical fiber manufacturingand installation for a system which provides predictive PMD throughoutthe manufacturing and installation process.

SUMMARY OF THE INVENTION

In accordance with one implementation consistent with the principles ofthe invention, a method is provided for predicting an installedperformance parameter of an optical fiber cable. The method includesobtaining a measurement indicative of a value of the performanceparameter at a first moment in time. A measurement indicative of a valueof the performance parameter at a second moment in time may be obtained.A first correlation may be determined between the measurement at thefirst moment in time and the measurement at the second moment in time. Avalue of the performance parameter at the second moment in time may beestimated based upon the measurement at the first moment in time incombination with the first correlation, the first correlation beingbased upon observations of a manner in which the performance parametervaries over time for at least a second optical fiber.

In another implementation consistent with the principles of theinvention, a system is provided for predicting an installed polarizationmode density (PMD) of an optical fiber cable. The system includes meansfor obtaining a measurement indicative of a PMD value at a first momentin time. Means are provided for obtaining a measurement indicative of aPMD value at a second moment in time. Means are provided for determininga first correlation between the measurement at the first moment in timeand the measurement at the second moment in time. Means are provided forpredicting a PMD value at the second moment in time based upon themeasurement at the first moment in time and the first correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an implementation of theinvention and, together with the description, explain the invention. Inthe drawings,

FIG. 1 is a block diagram of an exemplary fiber manufacturing system inwhich methods and systems consistent with the present invention may beimplemented;

FIG. 2 is an exemplary flow diagram, consistent with the presentinvention, illustrating one method for providing PMD mapping andanalysis through the entire fiber manufacture and installation process;

FIGS. 3 a, 3 b and 3 c are exemplary flow diagrams, consistent with thepresent invention, illustrating another method for providing PMD mappingand analysis through the entire fiber manufacture and installationprocess; and

FIG. 4 is an exemplary flow diagram, consistent with the presentinvention, illustrating one method for providing PMD prediction.

DETAILED DESCRIPTION

The following detailed description of implementations consistent withthe present invention refers to the accompanying drawings. The samereference numbers in different drawings may identify the same or similarelements. Also, the following detailed description does not limit theinvention. Instead, the scope of the invention is defined by theappended claims and equivalents.

Systems and methods consistent with the present invention providepolarization mode dispersion control during optical fiber manufacturing.According to one implementation consistent with principles of theinvention, measurements indicative of a value of the performanceparameter is obtained at first and second moments in time. A correlationbetween the measurements is then determined. A value of the performanceparameter at the second moment in time is then estimated based upon themeasurement at the first moment in time in combination with thecorrelation.

Exemplary Fiber Manufacturing System

FIG. 1 is a block diagram of an exemplary fiber manufacturing system 100in which methods and systems consistent with the present invention maybe implemented. System 100 may include a preform fabrication assembly102, a furnace 104, a diameter measuring meter 106, a coating applicator108, a coating curing system 110, a second diameter measuring meter 116,a winding mechanism 118, and a winding drum 120.

Manufacture of optical fiber is a precise, highly technical andspecialized process, resulting in fiber composed of two basic concentricglass structures: a core, which carries the light signals, and cladding,which traps the light in the core. As illustrated in FIG. 1, there arethree main steps in the process of converting raw materials into opticalfiber ready to be shipped. Initially, preform fabrication assembly 102may manufacture a pure glass preform. Next, winding mechanism 118 maydraw the preform into a hair thin fiber through furnace 104, coatingapplicator 108, cured system 110, and the diameter measuring meters 106and 116. Lastly, the fiber is taken up on a winding drum 120 for cuttingand eventual shipping to cable manufacturers.

The first step in manufacturing glass optical fibers is to make thesolid glass preform. There are several methods for forming preforms,such as Modified Chemical Vapor Deposition (MCVD), Plasma ModifiedChemical Vapor Deposition (PMCVD), Plasma Chemical Vapor Deposition(PCVD), Outside Vapor Deposition (OVD), and Vapor-phase Axial Deposition(AVD). In MCDV, ultra-pure chemicals, such as primarily silicontetrachloride (SiCl₄) and germanium tetrachloride (GeCl₄), are burned inoxygen during preform manufacturing. Resulting oxides and silica arethen sintered at high temperature. These chemicals are used in varioustypes and proportions to fabricate the core regions for the differenttypes of preforms and fibers.

The mixtures of chemicals described above are carried to the inside of arotating glass starting tube made of pure synthetic SiO₂ (silica). Thepure silica tube is mounted on a rotating lathe equipped with a specialheat torch which traverses the length of the lathe at a predeterminedspeed and temperature. As the gasses flow inside the tube, they react tothe heat by forming solid submicron particles, called “soot,” as thetorch passes. Once formed, the soot is deposited on the inner wall ofthe rotating tube. As the burner traverses over the deposited soot, theheat transforms the soot particles into pure, transparent glass, in aprocess called vitrification. The process is typically repeated for manyhours as each subsequent core layer is formed. After the desired amountof core material has been deposited the chemical flow is eliminated. Thespeed of the torch may then be decreased and the temperature of theflame may be increased so that the tube collapses into a solid rod.

In accordance with principles of the invention, PMD for the preform ismeasured at the preform fabrication assembly 102. It should beunderstood that PMD may be measured in any suitable manner. Examples ofmeasurement techniques include, but are not limited to, the Jones MatrixEigenanalysis (JME) method, wavelength scanning, and the interferometricmethod. Each method results in a PMD or expected PMD value representingthe delay between the two polarization orientations normalized over thelength of the fiber or preform. The units of PMD are picoseconds (ofdelay) per root kilometer of length (ps/{square root}km). Additionally,it should be understood that each PMD measurement typically includes aplurality of individual measurements, or samples, over a predeterminedtime period. A distribution or histogram of the measurements may begenerated to identify a mean PMD value for the stage and to moreaccurately reflect the stage's sensitivity to PMD fluctuation. Thedistribution may them be analyzed to identify a particular stage'ssensitivity or to identify process control issues resulting in unusualdistributions.

As will be discussed in additional detail below, a determination is thenmade regarding whether the measured value of PMD meets or exceeds apredetermined specification or threshold. In one implementation, thethreshold may be a confidence interval relating to the mean PMD valuereferenced above. If the preform's mean PMD falls outside of thedetermined threshold or confidence interval, the preform may be removedfrom manufacturing system 100 and analyzed to determine the reasonsbehind the deficient PMD value. In one implementation, a root causeanalysis (RCA) is performed on the preform to determine the cause of theelevated PMD value. Another implementation would be the distribution ofPMD along the perform length.

However, if the preform's mean PMD value falls within of the definedinterval, the next step in the process of producing optical fibers is toconvert the manufactured preform into a hair-thin fiber. This is done inan operation called fiber draw and typically is performed in a verticaltower utilizing gravity in the fiber draw process. Details regarding thespecifics of the fiber draw process have been described above inrelation to FIG. 1. In creating the fiber from the preform, the tip ofthe preform is lowered into a furnace 104. In one implementation,furnace 104 may be a high-purity graphite furnace. Pure gasses areinjected into furnace 104 to provide a clean and conductive atmosphere.Furnace 104 is heated to temperatures approaching 1900° C. so as tosoften the tip of the preform. Once the softening point of the preformtip is reached, gravity takes over and allows a molten gob to “freefall” until it has been stretched into a thin strand.

Once formed, the strand of fiber is measured by meter 106 and fedthrough a series of coating dies 108, 110, and the drawing processbegins. The fiber is measured by meter 116, pulled by winding mechanism118 situated at the bottom of the draw tower and then wound on windingdrum 120. Typically, winding mechanism 118 operates at draw speeds ofapproximately 10-20 meters per second.

During the draw process the diameter of the drawn fiber is controlled toa predetermined diameter, for example 125 microns. A diameter gauge maybe used to monitor the diameter of the fiber. In one implementation, thediameter gauge may be a laser-based micrometer. The actual value of thediameter may then be compared to the 125 micron target. Slightdeviations from the target may be converted to changes in draw speedsand fed to the winding mechanism 118 for correction.

A two layer protective coating may be then applied to the fiber, such asa soft inner coating and a hard outer coating in coating applicator 108and coating curing system 110. This two-part protective jacket providesmechanical protection for handling while also protecting the pristinesurface of the fiber from harsh environments. Coating curing system 110may use ultraviolet lamps to cure the coatings.

In accordance with principles of the invention, a PMD value is againmeasured at the drum stage. As will be discussed in additional detailbelow, a correlation between PMD values at the preform and drum stagesmay be determined indicating likely causes for poor PMD values.

Exemplary Processing

Once the fiber has been wound onto a winding drum it is ready forcutting, cabling and installation. In accordance with principles of theinvention, PMD measurements throughout the manufacturing andinstallation process together enable accurate mapping of fiber PMD. FIG.2 is an exemplary flow diagram, consistent with the present invention,illustrating one method for providing PMD mapping and analysis throughthe entire fiber manufacturing and installation process. Initially, aPMD measurement is made during the fiber manufacturing stage of anoptical fiber manufacturing and installation process (act 200). Inaccordance with one implementation consistent with the invention, thismeasurement may be made following preform formation by preformfabrication assembly 102.

As discussed above, a PMD measurement may include measuring a pluralityof samples and identifying a histogram or distribution and a mean PMDmeasurement of the stage. At this point, it is determined whether themeasured PMD meets or exceeds a predetermined threshold or confidenceinterval (act 202). Another implementation would be the PMD shape of thehistogram or distribution and its change over time. If the manufacturingstage PMD value does not meet or exceed the predetermined threshold, themanufactured fiber is removed from the manufacturing process andanalyzed for failure causes (act 204). However, if the threshold is met,a histogram is generated relating to the measured PMD value(s) (act206). The process may then continue to a cabling stage where theindividual fibers are bundled together to form a cable (act 208).

Next, a cabling stage PMD measurement is made (act 210). At this point,it may be determined whether the measured PMD value meets or exceeds apredetermined threshold (act 212). It should be understood that thethreshold of act 212 may differ from that of act 202 depending upon theexpected PMD value. If the cabling stage PMD value does not meet orexceed the predetermined threshold, the fiber or cable is re-measuredand/or analyzed to determine the cause for the failure (act 214). If thethreshold is met, a histogram may be generated relating to the measuredPMD value(s) (act 216). The process may then continue to an installedstage where the fiber cables are received in the field, spliced andinstalled (act 218).

An installed stage PMD measurement is made (act 220). It may then bedetermined whether the measured PMD value meets or exceeds apredetermined threshold (act 222). If the installed stage PMD value doesnot meet or exceed a predetermined threshold (which may again bedifferent that than the thresholds of acts 202 and 212, above), thefiber or cable is re-measured and analyzed to determine the cause forthe failure (act 224). If the threshold is met, a histogram may begenerated relating to the measured PMD value(s) (act 226).

Once PMD measurements for at least two stages have been conducted formultiple fibers or cables, a correlation between the two stages may beidentified. For example, in accordance with the present implementation,a correlation coefficient (r) may be identified between PMD measurementsmade during the manufacturing stage (act 200) and the PMD measurementsmade during the cabling stage (act 210) (act 228). This coefficient mayindicate a predictable shift in PMD between the stages. Additionally, asecond correlation coefficient may be identified between PMDmeasurements made during the cabling stage (act 210) and those madeduring the installed stage (act 220) (act 230).

In one implementation consistent with principles of the invention, thecorrelation coefficient is a Pearson Product Moment correlationcoefficient, wherein a perfect positive correlation is denoted by acorrelation coefficient of 1 and a perfect negative correlation isdenoted by a correlation coefficient of −1. Additionally, a square ofthe correlation coefficient (r²) provides a proportion of the varianceof a measurement at one stage explained by the variance at a secondstage. For example, a correlation coefficient of 0.8 (r²=0.64) indicatesthat 64% of the variation in PMD measurements at a second stage can beexplained by the variation in PMD measurements at the first stage.Although the Pearson correlation coefficient has been disclosed forexemplary purposes, it should be understood that any suitablecorrelation analysis may be conducted (e.g., Spearman correlationcoefficient, etc.).

In addition to a correlation coefficient, data measured during the fibermanufacturing and installation process may also be reviewed using aregression analysis to generate an equation representing a linearrelationship between measured PMD values. In one implementationconsistent with principles of the invention, a linear regression may beperformed identifying an equation having the form y=m×+b, where m is theslope of the identified line, and b is the intercept (the point wherethe line crosses the y axis). For this equation, the relative PMDmeasurements for a second stage may be predicted from a measurement at afirst stage. Analysis of this line may indicate changes in themanufacturing process or environment as well as sensitivity to PMDfluctuations. Included in this equation might be a “goodness of fit”which could be used as a measure of the “scatter” of PMD points on theline. If the scatter is too large, the manufacturing process might bere-examined for RCA.

In one implementation consistent with principles of the invention,correlation coefficients between various stages may then be thencombined to generate a generic correlation coefficient for the entireprocess (act 232). Once an initial set of correlation coefficientsand/or regression equations has been identified, an analysis of thecoefficients may be conducted to determine which stages most sensitivelyimpact the PMD of installed fibers (act 234). In accordance with animplementation of the invention, this may be done by analyzing thecorrelation coefficients, regression equations and histograms generatedabove. In this manner, subsequent measurements of PMD values may berestricted to the identified stage or stages. This enhances theefficiency of the overall process. Once the sensitive stages and theirrespective correlation coefficients have been identified, installed PMDvalues may be predicted for fibers/cables throughout themanufacturing/installation process (act 236). In one implementationconsistent with principles of the invention, installed PMD values may bepredicted using a linear equation or a set of linear equations generatedbased upon sample measurements from at least two stages, in the mannerdescribed above. Another enhancement might be a 3 dimensional graph orplot using 3 different stages of manufacture to show changes in shapeamongst the 3 different sensitive stages.

By measuring PMD values at a sensitive stage or group of stages,correlations between the measurements may be made and used to moreaccurately predict which cables will perform within specifications wheninstalled in the field from an earlier stage ofmanufacture/installation.

FIGS. 3 a, 3 b and 3 c are exemplary flow diagrams, consistent with thepresent invention, illustrating another method for providing PMD mappingand analysis through the entire fiber manufacturing and installationprocess.

As described above, optical fiber manufacturing begins with the creationof an initial fiber preform (act 300) (FIG. 3 a). Following preformcreation, a PMD measurement may be made using any suitable method,examples of which are provided above (act 302). At this point, it isdetermined whether the measured PMD meets or exceeds a predeterminedthreshold (act 304). If the preform PMD value does not meet or exceedthe predetermined threshold, the preform is removed from themanufacturing process and analyzed for failure causes (act 306).However, if the threshold is met, a histogram may be generated relatingto the measured PMD value(s) (act 308). The manufacturing process maythen continue through the fiber draw stage to the drum stage asdescribed above (act 310).

Once the manufactured fiber is wound onto a drum, a PMD measurement mayagain be made using any suitable method (act 312). At this point, it isdetermined whether the measured PMD meets or exceeds a predeterminedthreshold (act 314). It should be understood that the threshold of act314 may differ from that of act 304 depending upon the expected PMDvalue. If the drum stage PMD value does not meet or exceed thepredetermined threshold, a histogram may be created and the fiber may beremoved from the manufacturing process and analyzed for failure causes(act 316). As described in detail above, PMD measuring techniques maygenerate a number of sample measurements that may be collected in adistribution or histogram. A mean value for the histogram may representa measured PMD for the fiber. A predetermined confidence intervalsurrounding the mean may define the threshold of acceptable fibers.Also, the shape of the histogram and its change may be used as anindicator. In addition, the histograms generated for each stage may alsobe analyzed to identify manufacturing or cabling process or equipmentissues which may manifest themselves in unusual distributions.

If the threshold is met, another histogram may be generated relating tothe measured PMD value(s) (act 318). The process then continues to acutting stage, where the fiber is cut to appropriate shipping lengths(e.g., 8-20 km) (act 320).

Once the manufactured fiber has been cut to its shipping length, anotherPMD measurement is made (act 322). At this point, it is determinedwhether the measured PMD meets or exceeds a predetermined threshold (act324). As above, the threshold of act 324 may differ from that of acts304 and/or 314 depending upon the expected PMD value. If the cuttingstage PMD value does not meet or exceed the predetermined threshold, thefiber may be removed from the shipping/installation process and analyzedfor failure causes (act 326). Alternatively, the fiber may bere-measured. If the threshold is met, a histogram is created relating tothe PMD values measured in act 324 (act 328). The process may continueto a packaging/shipping stage, where the fiber is cut to appropriateshipping lengths and packaged for delivery (act 330).

Once the fiber has been delivered, another set of PMD measurements ismade (act 332). At this point, it is determined whether the measured PMDmeets or exceeds a predetermined threshold (which may again be differentfrom thresholds identified above) (act 334). If the incoming fiber PMDvalue does not meet or exceed the predetermined threshold, the fiber isre-measured to determine a cause for the failure (act 336). In oneimplementation, a per fiber correlation may be generated in comparisonto the measurements at the packing/shipping stage. If the threshold ismet, a histogram may be created relating to the PMD values measured inact 332 (act 338). The process may continue to a loose tubes stage,wherein the fiber has been installed into bundles in loose tubes (act340).

Once in loose tubes, another set of PMD measurements is made (act 342).At this point, it is again determined whether the measured PMD meets orexceeds a predetermined threshold (which may be the same or differentfrom previously tested thresholds) (act 344). If the incoming fiber PMDvalue does not meet or exceed the predetermined threshold, the fiber maybe re-measured to determine a cause for the failure (act 346). However,if the threshold is met, a histogram may be created relating to the PMDvalues measured in act 342 (act 348). The process may continue to astranding stage, where the fiber is stranded into a core cable (act350).

Once stranded into a core cable, another set of PMD measurements is made(act 352). At this point, it is again determined whether the measuredPMD meets or exceeds a predetermined threshold (which may be differentor the same as the various thresholds identified above) (act 354). Ifthe incoming fiber PMD value does not meet or exceed the predeterminedthreshold, the fiber may be re-measured to determine a cause for thefailure (act 356). However, if the threshold is met, a histogram may becreated relating to the PMD values measured in act 352 (act 358). Theprocess may continue to a cabling stage, where the fiber is finallycabled (act 360) (FIG. 3 b).

Once cabled, another set of PMD measurements is made (act 362). At thispoint, it is again determined whether the measured PMD meets or exceedsa predetermined threshold (which may be different or the same as thevarious thresholds identified above) (act 364). If the incoming fiberPMD value does not meet or exceed the predetermined threshold, the fibermay be re-measured to determine a cause for the failure (act 366).However, if the threshold is met, a histogram may be created relating tothe PMD values measured in act 362 (act 368). The process may continueto a shipping reel stage, where the fiber cables are mounted on shippingreels (act 370).

Once mounted on shipping reels, another set of PMD measurements is madefor the cabled fiber (act 372). At this point, it is again determinedwhether the measured PMD meets or exceeds a predetermined threshold(which may be different or the same as the various thresholds identifiedabove) (act 374). If the incoming fiber PMD value does not meet orexceed the predetermined threshold, the fiber may be re-measured todetermine a cause for the failure (act 376). However, if the thresholdis met, a histogram may be created relating to the PMD values measuredin act 372 (act 378). The process may continue to a shipped reel fieldmeasurement stage, where the shipped reels are measured in the field(act 380).

Once the shipping reels are in the field, another set of PMDmeasurements is made (act 382). At this point, it is again determinedwhether the measured PMD meets or exceeds a predetermined threshold(which may be different or the same as the various thresholds identifiedabove) (act 384). If the incoming fiber PMD value does not meet orexceed the predetermined threshold, the fiber may be re-measured todetermine a cause for the failure (act 386). However, if the thresholdis met, a histogram may be created relating to the PMD values measuredin act 382 (act 388). The process may continue to a pre-splicinginstalled stage, where the shipped reels are measured in the field (act390).

Once installed prior to splicing, another set of PMD measurements ismade for the pre-spliced cable (act 392). At this point, it is againdetermined whether the measured PMD meets or exceeds a predeterminedthreshold (which may be different or the same as the various thresholdsidentified above) (act 394). If the incoming fiber PMD value does notmeet or exceed the predetermined threshold, the fiber may be re-measuredto determine a cause for the failure (act 396). However, if thethreshold is met, a histogram may be created relating to the PMD valuesmeasured in act 392 (act 398). The process continues to a splicedinstalled stage, wherein the fiber cables are installed and spliced inthe field (act 400).

Once the fiber has been installed and spliced, another set of PMDmeasurements may be made for the cable (act 402). At this point, it isagain determined whether the measured PMD meets or exceeds apredetermined threshold (which may be different or the same as thevarious thresholds identified above) (act 404). If the incoming fiberPMD value does not meet or exceed the predetermined threshold, the fibermay be re-measured to determine a cause for the failure (act 406).However, if the threshold is met, a histogram may be created relating tothe PMD values measured in act 402 (act 408). The installation processis then complete and continues to post-install stage (act 410).

Once the fiber has been completely installed, another final set of PMDmeasurements is made for the cable (act 412). In practice, this mayrequire determination of a cable's Link Design Value (LDV) which, isgenerally defined as the 99.99 percentile of the PMD coefficient of theconcatenation of 20 cable sections that could be randomly drawn from theprocess distribution.

At this point, it is again determined whether the measured PMD (or LDV)meets or exceeds a predetermined threshold (act 414). If the PMD valuedoes not meet or exceed the predetermined threshold, the cable isre-measured to determine a cause for the failure (act 416). However, ifthe threshold is met, a histogram is created relating to the PMD valuesmeasured in act 412 (act 418).

Once PMD measurements for at least two stages have been conducted, acorrelation between the two stages may be identified. For example, inaccordance with the present implementation, a correlation coefficient(r) may be identified (e.g., by the Pearson correlation coefficientmethod described above) between PMD measurements made during the preformstage (act 302) and the PMD measurements made during the drum stage (act312) (act 420) (FIG. 3 c). This coefficient may indicate a predictableshift in PMD between the stages. Additionally, correlation coefficientsmay be identified between: the cutting stage and the packaging/shippingstage (act 422); the incoming fiber stage and the loose tubes stage (act424); the loose tubes stage and the core cable stage (act 426); thecabling stage and the shipping reel stage (act 428); the shipped reel inthe field stage and the pre-splicing installed stage (act 430); and thespliced installed stage and the final installed stage (act 432).

Although specific stage combinations have been set forth above, itshould be understood that any combination of measurement stages may beemployed to generate the correlation coefficients. For example, acorrelation coefficient may be generated using PMD measurements fromloose tubes stage and the PMD measurements from the final splicedinstalled stage. In this manner, specific indications of stagecorrelation in relation to PMD fluctuations may be ascertained. Forexample, measurements at the incoming fiber stage and the pre-splicinginstalled stage may indicate a strong correlation. Additionally, asdescribed above, regression equations may also be generated for each setof stages, illustrating expected values of PMD for each stage relativeto another. Furthermore, it should be understood that correlations maybe performed for more than two stages resulting in multi-dimensionalcorrelations. Correlation coefficients between the various stages arethen combined to generate a generic correlation coefficient for theentire process (act 434).

FIG. 4 is an exemplary flow diagram, consistent with the presentinvention, illustrating one method for predicting PMD for installedfiber applications. Initially, the various histograms, correlationcoefficients and regression equations generated in FIGS. 3 a-3 c may beanalyzed (hereinafter the “sensitivity analysis”) to identify thosestages having the greatest sensitivity regarding their effect oninstalled PMD for the bundled fiber (act 500). In this manner,measurements and correlations for non-sensitive stages may be minimized,thereby enhancing the efficiency of the PMD prediction process.Additionally, by reducing the number of less sensitive measurementsincluded within the analysis, scatter is effectively reduced, therebyenhancing the accuracy and reliability of any prediction generated. Inaccordance with one implementation of the invention, the sensitivityanalysis may be based on various quantifiable criteria, such as changesto PMD histograms (and/or histogram shapes) from each stage, value ofcorrelation coefficients associated with the stage, relative fluctuationin linear regression equations generated for the stage, etc.

Once a sensitive stage or stages has been identified, correlationcoefficients and/or regression equations between the identified stagesare generated (act 502). Once the correlations and/or equations of act502 have been identified, installed PMD values may be predicted forfibers/cables throughout the manufacturing/installation process (act504).

As described above, by measuring PMD values at a plurality of stagesthroughout the cable manufacturing and installation process,correlations between the measurements may be made and used to moreaccurately predict which cables will perform within specifications wheninstalled in the field from an earlier stage ofmanufacture/installation.

CONCLUSION

Implementations consistent with the principles of the invention provideorganizations with an efficient and effective process for performingperformance parameter control and analysis. More particularly, in oneimplementation consistent with principles of the invention, measured PMDvalues may be correlated at various times throughout the optical fibermanufacturing process to enable predicted PMD values for installedcable. Additionally, although the above description explicitly discussesPMD as the measure performance parameter, it should be understood thatthe instant methodology may be applicable to numerous optical fiberperformance parameters, such as the entire spectral insertion loss (O-Lbands), especially at the micro bending edge (1565 nm), attenuation,dispersion, and any other pertinent measured (or unmeasured) fiberparameter and other such parameters which are difficult to track throughoptical fiber and cable the manufacturing process.

The foregoing description of exemplary embodiments of the presentinvention provides illustration and description, but is not intended tobe exhaustive or to limit the invention to the precise form disclosed.Modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention.

Moreover, while series of acts have been described with regard to FIGS.3 a, 3 b, 3 c and 4 the order of the acts may be varied in otherimplementations consistent with the present invention. In addition,non-dependent acts may be implemented in parallel.

No element, act, or instruction used in the description of the presentapplication should be construed as critical or essential to theinvention unless explicitly described as such. Also, as used herein, thearticle “a” is intended to include one or more items. Where only oneitem is intended, the term “one” or similar language is used. Further,the phrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise. The scope of the invention isdefined by the claims and their equivalents.

1. A method for predicting an installed performance parameter of anoptical fiber cable, comprising: obtaining a measurement indicative of avalue of the performance parameter at a first moment in time; obtaininga measurement indicative of a value of the performance parameter at asecond moment in time; determining a first correlation between themeasurement at the first moment in time and the measurement at thesecond moment in time; and estimating a value of the performanceparameter at the second moment in time based upon the measurement at thefirst moment in time in combination with the first correlation, thefirst correlation being based upon observations of a manner in which theperformance parameter varies over time for at least a second opticalfiber.
 2. The method of claim 1, wherein the first moment in time occursduring manufacture of the optical fiber cable and the second moment intime occurs subsequent to manufacture of the optical fiber cable.
 3. Themethod of claim 1, further comprising: obtaining a measurementindicative of a value of the performance parameter at an nth moment intime, where n is an arbitrary integer and wherein the nth moment in timerepresents any moment in time; and determining a second correlationbetween a first measurement selected from the measurement at the firstmoment in time, the second moment in time, or the nth moment in time anda second measurement selected from the measurement at the first momentin time, the second moment in time, or the nth moment in time.
 4. Themethod of claim 1, further comprising: determining the secondcorrelation between the measurement at the second moment in time and ameasurement at a third moment in time.
 5. The method of claim 3, furthercomprising: estimating a value of the performance parameter at an mthmoment in time based upon the measurement at the first moment in timeand the first and second correlations, where m is an arbitrary integer.6. The method of claim 3, further comprising: identifying at least onesensitive moment in time from the first, second and nth moments in time;and estimating a value of the performance parameter at an mth moment intime based upon the measurement at the sensitive moment in time and thefirst, second and third correlations, where m is an arbitrary integer.7. The method of claim 3, further comprising: determining a genericcorrelation using the first and second correlations; and estimating avalue of the performance parameter at an mth moment in time based uponthe generic correlation, where m is an arbitrary integer.
 8. The methodof claim 3, wherein the first moment in time occurs during manufactureof the optical fiber cable, the second moment in time occurs during thecabling of the optical fiber cable, and the nth moment in time occursduring installation of the optical fiber cable.
 9. The method of claim1, further comprising: determining if the measurement indicative of thevalue of the performance parameter at the first moment in time meets orexceeds a first predetermined threshold; removing the optical fibercable if it is determined that the measurement indicative of the valueof the performance parameter at the first moment in time does not meetor exceed the first predetermined threshold; and analyzing the opticalfiber cable to determine a root cause for failure.
 10. The method ofclaim 1, further comprising: generating a first histogram indicative ofthe measurement at the first moment in time; and generating a secondhistogram indicative of the measurement at the second moment in time.11. The method of claim 1, further comprising: generating a square ofthe correlation coefficient to provide a proportion of a variance of themeasurement at the second moment in time explained by a variance at thefirst moment in time; and estimating the value of the performanceparameter at the second moment in time based upon the measurement at thefirst moment in time in combination with the first correlation and thesquare of the correlation coefficient.
 12. The method of claim 1,wherein the measurement at the first moment in time includes a pluralityof measurements at the first moment in time; and the measurement at thesecond moment in time includes a plurality of measurements at the secondmoment in time.
 13. The method of claim 12, further comprising:generating a regression equation representing a linear relationshipbetween the plurality of measurements at the first moment in time andthe plurality of measurements at the second moment in time; andestimating the value of the performance parameter at the second momentin time based on a subsequent measurement at the first moment in time incombination with the regression equation.
 14. The method of claim 1,wherein the performance parameter is polarization mode dispersion (PMD).15. The method of claim 14, wherein PMD is measured using a Jones MatrixEigenanalysis (JME) method.
 16. The method of claim 14, wherein PMD ismeasured using a wavelength scanning method.
 17. The method of claim 14,wherein PMD is measured using a interferometric method.
 18. A system forpredicting an installed polarization mode density (PMD) of an opticalfiber cable, comprising: means for obtaining a measurement indicative ofa PMD value at a first moment in time; means for obtaining a measurementindicative of a PMD value at a second moment in time; means fordetermining a first correlation between the measurement at the firstmoment in time and the measurement at the second moment in time; andmeans for predicting a PMD value at the second moment in time based uponthe measurement at the first moment in time and the first correlation.19. The system of claim 18, further comprising: means for obtaining ameasurement indicative of a PMD value at a third moment in time; meansfor determining a second correlation between the measurement at thesecond moment in time and the measurement at the third moment in time;and means for estimating a value of the performance parameter at thethird moment in time based upon the measurement at the first moment intime and the first and second correlations.
 20. The system of claim 18,further comprising: means for determining if the measurement indicativeof the PMD value at the first moment in time meets or exceeds a firstpredetermined threshold; means for discarding the optical fiber cable ifit is determined that the measurement indicative of the PMD value at thefirst moment in time does not meet or exceed the first predeterminedthreshold; and means for analyzing the optical fiber cable to determinea root cause for failure.